Center Of Pressure Time Series Research Articles

Between-limb synchronization for control of standing balance in individuals with stroke

Background During standing, forces and moments exerted at the feet serve to maintain stability in the face of constant centre-of-mass movement. These actions are temporally synchronized in healthy individuals. Stroke is typically a unilateral injury resulting in increased sensori-motor impairment in the contra-lesional compared with the ipsi-lesional lower-limb, which could lead to reduced between-limb synchronization for control of standing balance. The purpose of this study is to investigate between-limb synchronization of standing balance control in individuals with stroke; a potentially important index of control of upright stability. Methods Twenty healthy controls and 33 individuals with unilateral stroke were assessed. Stability was assessed during a 30-second quiet standing trial by measuring data from two force plates (one per foot). Limb-specific centre of pressure was calculated. Between-limb synchronization was defined as the coefficient of the correlation between the left and right foot for both the antero-posterior and medio-lateral centre of pressure time series. Synchronization, weight-bearing symmetry, and root mean square of the total centre of pressure excursion were compared between controls and stroke participants. Findings Stroke participants swayed more, were more asymmetric, and had less between-limb synchronization than healthy controls. Among individuals with stroke, reduced between-limb synchronization was related to increased postural sway in the medio-lateral direction and increased weight-bearing asymmetry. Interpretation Individuals with stroke have reduced temporal synchronization of centre of pressure fluctuations under the feet when controlling quiet standing. The clinical significance of reduced synchronization remains to be determined, although it appears linked to increased medio-lateral sway and weight-bearing asymmetry.

To freeze or not to freeze? Affective and cognitive perturbations have markedly different effects on postural control

Similar effects have been reported for diverting attention from postural control and increased anxiety on the characteristics of center-of-pressure (COP) time series (decreased excursions and elevated mean power frequency). These effects have also received similar interpretations in terms of increased postural stiffness, suggesting that cognitive and affective manipulations have similar influences on postural control. The present experiment tested this hypothesis by comparing postural conditions involving manipulations of attention (diverting attention from posture using cognitive and motor dual tasks) and anxiety (standing at a height), and by complementing posturography with electromyographic analyses to directly examine neuromuscular stiffness control. Affective and cognitive manipulations had markedly different effects. Unlike the height condition, diverting attention from balance induced smaller COP amplitudes and higher sway frequencies. In addition, more regular COP trajectories (lower sample entropy) were found in the height condition than the dual-task conditions, suggesting elevated attentional investment in posture under the affective manipulation. Finally, based on an analysis of the cross-correlation function between anterior–posterior COP time series and enveloped calf muscle activity, indications of tighter anticipatory neuromuscular control of posture were found for the height condition only. Our data suggest that affective and cognitive perturbations have qualitatively different effects on postural control, and thus are likely to be associated with different control processes, as evidenced by differences in neuromuscular regulation and attentional investment in posture.

Posturographic measures in healthy young adults during quiet sitting in comparison with quiet standing

Measures of postural steadiness – known as posturography – are commonly used for balance assessment during quiet standing. Although quiet sitting balance may be studied via posturography as well, this has not been done to date. As such, the purpose of this study was to characterize the posturography during quiet sitting in comparison with quiet standing and to provide a benchmark for future studies investigating differences in balance regulation and execution. Twelve young and healthy people agreed to quietly sit and stand on a force platform with their eyes open and closed. For each condition, one trial of 2 min was executed and the anterior–posterior, medial–lateral, and resultant distance fluctuations of the body's center of pressure (COP) were calculated. Finally, time-domain, frequency-domain, and stabilogram diffusion function (SDF) measures were identified and compared for all COP time series. The results consistently indicate that, for quiet sitting, the body sway size and velocity were smaller and the power-weighted average frequency larger than for quiet standing. Moreover, the SDF analysis revealed that quiet sitting shows fewer drifts over short time intervals, but also fewer controlled adjustments in the longer term to bring the system back to equilibrium. The observed differences can be partially explained by biomechanical and dynamic differences of the body portions that are in motion during quiet sitting and standing. The SDF analysis suggests, however, that also the balance control strategies are not identical. These findings may be especially useful for the assessment of sitting balance and the development of novel balance rehabilitation techniques and assistive devices.

Estimation procedures affect the center of pressure frequency analysis

Even though frequency analysis of body sway is widely applied in clinical studies, the lack of standardized procedures concerning power spectrum estimation may provide unreliable descriptors. Stabilometric tests were applied to 35 subjects (20-51 years, 54-95 kg, 1.6-1.9 m) and the power spectral density function was estimated for the anterior-posterior center of pressure time series. The median frequency was compared between power spectra estimated according to signal partitioning, sampling rate, test duration, and detrending methods. The median frequency reliability for different test durations was assessed using the intraclass correlation coefficient. When increasing number of segments, shortening test duration or applying linear detrending, the median frequency values increased significantly up to 137%. Even the shortest test duration provided reliable estimates as observed with the intraclass coefficient (0.74-0.89 confidence interval for a single 20-s test). Clinical assessment of balance may benefit from a standardized protocol for center of pressure spectral analysis that provides an adequate relationship between resolution and variance. An algorithm to estimate center of pressure power density spectrum is also proposed.

An overview of age-related changes in postural control during quiet standing tasks using classical and modern stabilometric descriptors

Age-related changes in postural control during quiet standing likely result from underlying pathological conditions or from the low specificity of classical stabilometric parameters, which are vulnerable to base of support configurations and anthropometric differences. This study focuses on the identification of changes in postural control with natural aging by using conventional and recent stabilometric analysis, and on the interpretation of the stabilometric parameters according to a recently proposed framework of postural control. Quiet standing stabilometric tests were applied to 57 subjects equally divided into young, middle-aged and aged groups (19–29, 38–51 and 65–73 years, respectively) with eyes open and closed conditions. In addition to estimation of classical descriptors, center of pressure time series were approached according to a diffusion-like process and the recently proposed sway density curve. Two out of 10 estimated descriptors identified between-group differences. Aged subjects exhibited higher sway frequencies, possibly resulting from the increase of torque bursts produced by the plantar flexors, and stronger negative correlation between consecutive center of pressure displacements observed for long time intervals, likely due to higher amplitude of plantar flexors torque. Aging itself does not result in major changes of postural stability, but reflects a small increase in plantar flexion torque amplitude and frequency of torque adjustments, probably to compensate for the lower stiffness of calf muscle tendon in aged subjects.

Frequencies in Center of Pressure Time Series above 1Hz during Quiet Upright Stance Reflect the Use of a Hip Strategy

Upright stance is primarily maintained through the use of ankle and hip strategies, involving respective rotations about the two joints. Choice of and coordination between these two strategies is regulated by the central nervous systems (CNS). Postural control mechanisms used to maintain upright stance are often assessed (or inferred) using a spectral decomposition of center of pressure (COP) time series. Existing evidence, however, is contradictory in some respects. Specifically, there is conflicting evidence whether COP frequencies above 1.0 Hz reflect CNS function or use of a hip strategy. In this study, fatigue was induced to the lumbar extensors and shoulder flexors through intermittent sub-maximal (60% MVC) isotonic exercises, and used to ‘perturb’ postural control. Use of hip strategies was assessed by entropy methods, which indicates the level of ‘complexity’. Coordination between ankle and hip strategies was analyzed using a coherence method. Both power spectrum and Tsallis and entropies showed that the medio-lateral (ML) hip strategy was impaired by lumbar extensor fatigue. Coherence (i.e. coordination) between ankle and hip motions was reduced following fatigue in both muscles groups. Integrated with earlier evidence, these results indicate that, during quiet upright stance, COP frequencies above 1.0 Hz correspond to the hip strategy rather than reflecting CNS function in postural control.

Effects of external loads on balance control during upright stance: Experimental results and model-based predictions

The purpose of this study was to identify the effects of external loads on balance control during upright stance, and to examine the ability of a new balance control model to predict these effects. External loads were applied to 12 young, healthy participants, and effects on balance control were characterized by center-of-pressure (COP) based measures. Several loading conditions were studied, involving combinations of load mass (10% and 20% of individual body mass) and height (at or 15% of stature above the whole-body COM). A balance control model based on an optimal control strategy was used to predict COP time series. It was assumed that a given individual would adopt the same neural optimal control mechanisms, identified in a no-load condition, under diverse external loading conditions. With the application of external loads, COP mean velocity in the anterior–posterior direction and RMS distance in the medial–lateral direction increased 8.1% and 10.4%, respectively. Predicted COP mean velocity and RMS distance in the anterior–posterior direction also increased with external loading, by 11.1% and 2.9%, respectively. Both experimental COP data and model-based predictions provided the same general conclusion, that application of larger external loads and loads more superior to the whole body center of mass lead to less effective postural control and perhaps a greater risk of loss of balance or falls. Thus, it can be concluded that the assumption about consistency in control mechanisms was partially supported, and it is the mechanical changes induced by external loads that primarily affect balance control.

Acute Effects of Aquatic Exercise on Postural Sway in Women with Lower Extremity Arthritis

PURPOSE: It is well known that individuals with lower extremity arthritis have difficulty in postural control. Previous studies showed long-term effects of aquatic exercise on their postural control. Our recent study investigated acute effects of aquatic exercise on their postural control and found that time domain measurements of center of pressure (COP), such as total sway area, did not change after an aquatic exercise session. In this study, we investigated its acute effects on frequency domain measurements of COP. METHODS: COP while in upright position of 7 female individuals with lower extremity arthritis (60.6 ± 6.13 yrs) was measured before and after an aquatic exercise session. Written informed consent was obtained from all subjects. The exercise session was 70 min that included 20 min warm up and walking, 15 min resistance exercise and stretching in water, 10 min break, 20 min walking or swimming and 5 min cooling down. COP was recorded for 30 sec with eyes open. Frequency domain measurements (total power, mean frequency (Hz), 50% power frequency (Hz) and 95% power frequency (Hz)) of COP and velocity time series of COP in anterior-posterior and medial-lateral directions were analyzed and the pre-and the post-exercise data were statistically compared using paired t-test. RESULTS: There were significant differences (p<0.05) in 95% power frequency of COP in anterior-posterior direction, mean frequency of velocity time series in both directions and 95% power frequency of velocity time series in medial-lateral direction; these variables shifted to lower frequency after an aquatic exercise session. CONCLUSION: It was showed that compared with young and/or healthy populations, older individuals and patients with Parkinson disease or spinocerebellar degeneration had less ability to keep postural stability as well as their power spectrum of COP and velocity time series showed more power in higher frequency area. Therefore, our results focusing on spectral characteristics of COP suggested that postural control of women with lower extremity arthritis was improved by the acute effects of aquatic exercise.

A Comprehensive Approach To The Center Of Pressure Differences Between Yoginis And Controls

Standard measures such as root mean square (RMS) and mean power frequency (MPF) adequately examine the magnitude of the center of pressure (COP), while nonlinear measures examine the structure of the COP time series. Combining these measures provides a comprehensive analysis of balance control (e.g. comparison of balance of dancers and track athletes, Schmit, Regis, and Riley, 2005). Increased COP amplitude could be due to decreased balance skills or increased exploratory behavior; nonlinear analyses can help differentiate between these interpretations. We expect that yoga practitioners will demonstrate different balance strategies when compared to controls. PURPOSE: To compare balance of yoginis and controls with traditional measures and non linear measures of COP. METHODS: Twenty females participated (27.9 ± 11.48 yrs.), ten yoginis and ten controls. Two randomly-presented focus conditions were observed (1) no instruction, where the participant stood quietly, (2) external fixation, where the participant was asked to focus on an × placed approximately three feet in front of them on the floor. Subjects stood either on two legs or one leg. RESULTS: The results for the anterior posterior sway direction are reported. No three way interactions (group × leg × vision) or two way interactions (group × leg; group × vision; leg × vision) were observed for the standard or the nonlinear measures. The standard measures revealed main effects for RMS for leg and vision conditions (p<.01 leg and p<.01 vision) as well as for MPF for leg condition (p<.01). The nonlinear measures revealed main effects for entropy across groups (p<.01), percent recurrence and percent determinism for the leg condition (p<.01). CONCLUSION: The magnitude and the frequency of the COP measure was not different across the two groups, however, the yoga group COP was less chaotic (decreased entropy) and had trends of increased noise (decreased percent recurrence). These changes are consistent with increased behavioral flexibility. A more complete understanding of postural control was observed when linear and non-linear measures were combined.

Kinematics Analysis of Knee Joint Motion in Closed Chain Movement among Different Sports Events: A Pilot Study

Accelerometry (ACC) shows promise as an easily implemented clinical measure of balance. The purpose of the study was to estimate test–retest reliability of ACC measures and determine the relationship between ACC measured at the pelvis and underfoot center of pressure (COP) measures during sensory organization test (SOT) conditions. Eighty-one subjects were recruited from the community with no known orthopedic or vestibular deficits (19–85 years). Subjects completed three consecutive, ninety second trials for each of the six SOT conditions, while wearing the accelerometer. ACC and COP time series were described by calculating the normalized path length, root mean square (RMS), and peak-to-peak values. The test–retest reliability of the three measures within each SOT condition was estimated over three trials using the intraclass correlation coefficient. ACC and COP test–retest reliability were similar, ranging from 0.63 to 0.80 using ACC and 0.42 to 0.81 using COP for the measure of normalized path length. Linear regression between ACC and COP measures showed significant correlation under almost every SOT condition using both single and average measures across trials. The degree of association between COP and ACC was equivalent when using the first trial or the 3-trial average, suggesting that one trial may be sufficient. The use of accelerometry may have value in estimating balance function and minimizing clinical evaluation time.

Feature selection of stabilometric parameters based on principal component analysis.

This study addresses the challenge of identifying the features of the Centre of pressure (COP) trajectory that are most sensitive to postural performance, with the aim of avoiding redundancy and allowing a straightforward interpretation of the results. Postural sway in 50 young, healthy subjects was measured by a force platform. Thirty-seven stabilometric parameters were computed from the one-dimensional and two-dimensional COP time series. After normalisation to the relevant biomechanical factors, by means of multiple regression models, a feature selection process was performed based on principal component analysis. Results suggest that COP two-dimensional time series can be primarily characterised by four parameters, describing the size of the COP path over the support surface; the principal sway direction; and the shape and bandwidth of the power spectral density plot. COP one-dimensional time series (antero-posterior (AP) and medio-lateral (ML)) can be characterised by six parameters describing COP dispersion along the AP direction; mean velocity along the ML and AP directions; the contrast between ML and AP regulatory activity; and two parameters describing the spectral characteristics of the COP along the AP direction. On the basis of the results obtained, some guidelines are suggested for the choice of stabilometric parameters to use, with the aim of promoting standardisation in quantitative posturography.

Comparison of three methods to estimate the center of mass during balance assessment

Evaluation of postural control is generally based on the interpretation of the center of pressure (COP) and the center of mass (COM) time series. The purpose of this study is to compare three methods to estimate the COM which are based on different biomechanical considerations. These methods are: (1) the kinematic method; (2) the zero-point-to-zero-point double integration technique (GLP) and (3) the COP low-pass filter method (LPF). The COP and COM time series have been determined using an experimental setup with a force plate and a 3D kinematic system on six healthy young adult subjects during four different 30 s standing tasks: (a) quiet standing; (b) one leg standing; (c) voluntary oscillation about the ankles and (d) voluntary oscillation about the ankles and hips. To test the difference between the COM trajectories, the root mean square (RMS) differences between each method (three comparisons) were calculated. The RMS differences between kinematic–LPF and GLP–LPF are significantly larger than kinematic–GLP. Our results show that the GLP method is comparable to the kinematic method. Both agree with the unified theory of balance during upright stance. The GLP method is attractive in the clinical perspective because it requires only a force plate to determine the COP–COM variable, which has been demonstrated to have a high reliability.

Nonlinear analysis of the development of sitting postural control.

The development of sitting postural control in five normal infants was examined longitudinally at three stages of sitting: Stage 1, when infants could hold up their head and upper trunk, but could not sit independently; Stage 2, when infants began to sit independently briefly; and Stage 3, when infants could sit independently. Methods from nonlinear dynamics were used to analyze center of pressure (COP) data during sitting in terms of stability of the neuromuscular system (Lyapunov Exponent), movement dimensionality (Correlation Dimension), and complexity/regularity (Approximate Entropy). Results indicated significant changes in the nonlinear measures over time, with increased stability and increased regularity revealing a more stable and periodic strategy of maintaining postural control. Dimensionality decreased from Stage 1 to 2, indicating a constraint of the degrees of freedom. Subsequently, dimensionality increased from Stage 2 to 3, indicating a release of the degrees of freedom as sitting independence emerged. Nonlinear analysis of the COP time series supports the perspective that the development of postural control is a dynamic process whereby the infant learns to control the body's degrees of freedom to achieve the sitting posture.

Noise-enhanced balance control in older adults.

Somatosensory information is critical to balance control and fall prevention in older adults. Recently, it has been shown that low-level input noise (electrical or mechanical) can enhance the sensitivity of the human somatosensory system. In this study, we tested the effect of low-level electrical noise, applied at the knee, on balance control in 13 healthy elderly volunteers. Subjects performed multiple single-legged stance trials with imperceptible electrical noise applied at the knee during half of the trials. Balance performance was characterized using a force platform to measure the displacement of the center of pressure (COP) under the subject's stance foot. Seven sway parameters were extracted from the COP time series. Improved balance was defined as a reduction in postural sway as indicated by decreases in the COP measures. Six of the seven sway parameters decreased with electrical noise. Three of these parameters decreased significantly ( < 0.05), and a fourth parameter was borderline significant. Averaged across subjects, the application of electrical noise resulted in a 3.8% reduction in mediolateral COP standard deviation ( = 0.04), a 5.4% decrease in the maximum anteroposterior COP excursion ( = 0.03), a 3.1% reduction in the COP path length ( = 0.04), and a 7.8% decrease in swept area ( = 0.05). The results suggest that imperceptible electrical noise, when applied to the knee, can enhance the balance performance of healthy older adults. These findings suggest that electrical noise-based devices may be effective in improving balance control in elderly people.

Spectral characteristics of visually induced postural sway in healthy elderly and healthy young subjects

Spectral analysis and time-frequency analysis were applied to anterior-posterior (A-P) center-of-pressure (COP) data collected during quiet stance and sinusoidal (0.25 Hz) moving visual scene perturbations from sixteen healthy elderly subjects and thirteen healthy young subjects. While the total energy of COP was larger in the elderly subjects than in the young subjects, energy-normalized spectra of the entire COP time series were similar between the two populations. Further time-dependent analysis of the spectral characteristics showed that the time-varying spectra were also similar, as revealed by time-frequency analysis, although some differences were observed. Specifically, time-dependent mean frequency and bandwidth responses to the stimulus showed similar increases in both young and elderly subjects with the introduction of the stimulus. Furthermore, time-frequency analysis showed that both groups exhibited an initial increase in sway at the stimulus frequency that then declined as the sinusoidal scene movement progressed. This reduction in sway during visual stimulation may indicate an "adaptation" to constant frequency visual perturbation that is invariant with age. This finding is in contrast to results reported recently for patients with vestibular impairments, who appear to lack such adaptation.

Rambling and trembling in quiet standing.

The goal of this study was to explore the rambling-trembling decomposition in quiet standing. The center of pressure (COP) and the horizontal ground reaction force (F(hor)) were registered in healthy subjects standing in an upright bipedal posture on a force platform. The COP positions at the instants when F(hor) = 0 were identified (instant equilibrium points, IEP) for the anterior-posterior direction, then the COP time series, were partitioned into its components using 2 different techniques, rambling-trembling decomposition and gravity line decomposition. The two decomposition techniques provided very similar results. An unexpectedly large correlation between the trembling trajectory and the difference between COP and gravity line was found, r = 0.91 (range, 0.83 < r < 0.98). The correlation implies that the GL moves from an IEP to the subsequent IEP along a smooth trajectory that can be predicted by the spline approximation. A substantial negative cross-correlation at a zero time lag was observed between the trembling and the F(hor), -0.90 < r < -0.75. For the rambling trajectory, the coefficients of correlation with F(hor) were low, -0.33 < r < -0.05. The data support the hypothesis that during quiet standing the body sways for two reasons: the migration of the reference point (rambling) and the deviation away from that point (trembling).

The effects of stochastic galvanic vestibular stimulation on human postural sway.

Galvanic vestibular stimulation serves to modulate the continuous firing level of the peripheral vestibular afferents. It has been shown that the application of sinusoidally varying, bipolar galvanic currents to the vestibular system can lead to sinusoidally varying postural sway. Our objective was to test the hypothesis that stochastic galvanic vestibular stimulation can lead to coherent stochastic postural sway. Bipolar binaural stochastic galvanic vestibular stimulation was applied to nine healthy young subjects. Three different stochastic vestibular stimulation signals, each with a different frequency content (0-1 Hz, 1-2 Hz, and 0-2 Hz), were used. The stimulation level (range 0.4-1.5 mA, peak to peak) was determined on an individual basis. Twenty 60-s trials were conducted on each subject - 15 stimulation trials (5 trials with each stimulation signal) and 5 control (no stimulation) trials. During the trials, subjects stood in a relaxed, upright position with their head facing forward. Postural sway was evaluated by using a force platform to measure the displacements of the center of pressure (COP) under each subject's feet. Cross-spectral measures were used to quantify the relationship between the applied stimulus and the resulting COP time series. We found significant coherency between the stochastic vestibular stimulation signal and the resulting mediolateral COP time series in the majority of trials in 8 of the 9 subjects tested. The coherency results for each stimulation signal were reproducible from trial to trial, and the highest degree of coherency was found for the 1- to 2-Hz stochastic vestibular stimulation signal. In general, for the nine subjects tested, we did not find consistent significant coherency between the stochastic vestibular stimulation signals and the anteroposterior COP time series. This work demonstrates that, in subjects who are facing forward, bipolar binaural stochastic galvanic stimulation of the vestibular system leads to coherent stochastic mediolateral postural sway, but it does not lead to coherent stochastic anteroposterior postural sway. Our finding that the coherency was highest for the 1- to 2-Hz stochastic vestibular stimulation signal may be due to the intrinsic dynamics of the quasi-static postural control system. In particular, it may result from the effects of the vestibular stimulus simply being superimposed upon the quiet-standing COP displacements. By utilizing stochastic stimulation signals, we ensured that the subjects could not predict a change in the vestibular stimulus. Thus, our findings indicate that subjects can act as "responders" to galvanic vestibular stimulation.

Interpretation of COM and COP balance control during quiet standing

Although several studies have applied traditional linear measures to evaluate postural control of patients with multiple sclerosis (MS), little is known about the nonlinear dynamics of this patient group. In this study, recurrence quantification analysis (RQA), a well documented nonlinear method, was used to compare the nonlinear dynamical structure of postural sway in two groups consisting of MS patients (n = 23) and healthy matched controls (n = 23). The study focuses on three levels of postural difficulty consisting of (1) standing on a rigid surface (force platform) with eyes open, (2) standing on a rigid surface with eyes closed, and (3) standing on a foam surface with eyes closed. The two levels of cognitive difficulty measured, consisted of a single postural task and a dual postural–cognitive task. It was observed that as the postural conditions became more difficult, the center of pressure (COP) time series of both groups became less regular as recorded in lower recurrence rate, less complex in deterministic structure as reflected in lower RQA entropy, and less nonstationary as reflected in the recording of lower Trend. Moreover, as cognitive conditions became more difficult, COP time series became less regular (lower %Rec in the anteroposterior direction and lower %Det in both directions), less complex in deterministic structure (lower RQA Ent in the anteroposterior direction), and less nonstationary (lower trend in the anteroposterior direction). The analytical results of the research show that there is a similar dynamical structure for both the MS patients and the control group; however, the nonlinear behavior of both groups was different under various experimental conditions.

On postural stability and variability

Abstract The stability of posture is often assessed directly by the variability of certain centre of pressure parameters, such as length or area of the centre of pressure profiles. Increased variability of these parameters is usually taken as an index of decreased postural stability. In this paper we present data from normal and tardive dyskinetic adult subjects that support the notion that centre of pressure variability is not a sufficient measure of postural stability. It is shown that variability in a given centre of pressure parameter does not necessarily distinguish the different attractor dynamics that support postural control. Dimension estimates of the centre of pressure time series showed that: (1) there is more structure in the centre of pressure pattern of normal subjects than is traditionally interpreted; (2) the dimension of the centre of pressure in tardive dyskinetic individuals is systematically lower than in normals. The implication of these findings for assessing the relation between the variability and stability of posture is discussed. It is proposed that postural stability can only be assessed by considering the attractor dynamics of the postural control system.